Ive never been to a site darker then bortle 3. One night at the WSP at about 4am after waking from a nap i thought it looked darker then normal blue zone skies im used to. Anyway is going from a blue zone with limiting magnitude of 6.9 to a bortle 2 worth the extra time and expense of adding a couple hours more driving into my trip this upcomng new moon weekend? Curious to hear from people who observe in grey zones and blue zones on the significance of improvement of views at the eyepiece between these two zones. Im really excited about potential grey zone trip but am afraid it may not be a noticeable difference. Ill still go if weather permits but would like to hear from experience what to expect. Thanks.

I would love to go to a blue zone, but wouldn't drive 2 extra hours for a gray. From what I recall reading in a different thread, the difference between Bortle 1-3 is only noticeable from 30* on down. Above 30*, the three are the same--very dark. Also, my guess is that a really good blue site in the wee hours of the night as you described, when the air is still and human activity is low (meaning more lights are off), it could be improve to meet the criteria of a gray zone (I think!). Best way to tell is to have an SQM on hand and measure darkness throughout the night.

I would love to go to a blue zone, but wouldn't drive 2 extra hours for a gray. From what I recall reading in a different thread, the difference between Bortle 1-3 is only noticeable from 30* on down. Above 30*, the three are the same--very dark.

Can you elaborate on what 30 degrees latitude has to do with it?

FWIW, my back yard is shown as light blue on the 16 color DarkSiteFinder map. Great Basin is shown as dark gray. My SQM-L readings for my yard are 21.5 MPSAS. The readings I took the other week on the only clear night of the Great Basin Star Party were also 21.5 MPSAS. My SQM-L readings in a dark closet are well over 23 MPSAS, so I don't think the meter was topping out in my back yard, but it does make me wonder about the meaning of blue, gray and dark gray zones, or the reliability of Sky Quality Meters.

Also FWIW, at Great Basin, I tried for, but couldn't see M33 with averted vision. This makes me suspicious of NELM listings, because they are all dependent on the quality of our individual eyes. As we get older, our lenses get cloudy, so an old guy's NELM isn't the same as a whippersnappers.

FWIW, my back yard is shown as light blue on the 16 color DarkSiteFinder map. Great Basin is shown as dark gray. My SQM-L readings for my yard are 21.5 MPSAS. The readings I took the other week on the only clear night of the Great Basin Star Party were also 21.5 MPSAS. My SQM-L readings in a dark closet are well over 23 MPSAS, so I don't think the meter was topping out in my back yard, but it does make me wonder about the meaning of blue, gray and dark gray zones, or the reliability of Sky Quality Meters.

Also FWIW, at Great Basin, I tried for, but couldn't see M33 with averted vision. This makes me suspicious of NELM listings, because they are all dependent on the quality of our individual eyes. As we get older, our lenses get cloudy, so an old guy's NELM isn't the same as a whippersnappers.

It's good to remember that the color zones for light pollution are only a rough guide, not an absolute indication of what every clear night will be like. Transparency, air pollution, aerosols, smoke, air glow and low-grade aurora make a huge difference night to night on how dark the sky will look and measure on an SQM. So there's no way to say that driving two more hours to an observing site will pay off with darker skies and better observing without having a good weather and space weather forecast a day or two in advance.

I would love to go to a blue zone, but wouldn't drive 2 extra hours for a gray. From what I recall reading in a different thread, the difference between Bortle 1-3 is only noticeable from 30* on down. Above 30*, the three are the same--very dark.

Can you elaborate on what 30 degrees latitude has to do with it?

He's referring to 30° above the horizon, below which most light domes occur.

I would love to go to a blue zone, but wouldn't drive 2 extra hours for a gray. From what I recall reading in a different thread, the difference between Bortle 1-3 is only noticeable from 30* on down. Above 30*, the three are the same--very dark.

Can you elaborate on what 30 degrees latitude has to do with it?

He's referring to 30° above the horizon, below which most light domes occur.

Yes, that is correct, I was referring to the horizon. The thread I was referring to is here and I guess it was only one comment, but no one refuted it and it certainly sounds plausible.

Starman - The statement made from the post you cite is, I'm afraid, decidedly off the mark with regard to accuracy when it is indicated that:

"Areas designated blue, grey or black are almost identical in appearance from 30° above the horizon to the zenith, in my experience. If fortunate enough to be observing from a clearing which is surrounded by tall, dense trees, which thoroughly block out any light domes that may be present on the horizon, one would be hard-pressed to discern much, if any, difference between these areas with one's eyesight alone."

The presence of light domes along the horizon impact the darkness of the entire sky to at least some degree, so simply blocking them out, together with hiding the lower 30* of sky, will not negate the differences between Bortle Classes 3, 2 and 1.

I would add that it is very unlikely that more than perhaps a handful of folks here have ever actually experience Bortle Class 1 skies. Believe me, if you ever have that opportunity, you'll have no doubt that decided differences exist between Bortle classes at the very lowest end of the scale.

Addressing the OP's question, the actual worth of driving considerably farther out from civilization to gain one or maybe two additional levels of darkness will depend critically on the nature of the observation you are attempting to make and perhaps the ultimate value that it may have to you and to others, at least in my book.

Transparency, air pollution, aerosols, smoke, air glow and low-grade aurora make a huge difference night to night on how dark the sky will look and measure on an SQM.

I've noticed varying levels of air glow on fairly transparent nights, but never to my knowledge have seen a "low grade aurora". At Lat 38 should I be looking for them?

Also, I just bookmarked the NOAA space weather page. Should I just be looking at the "Auroral Activity" link? If so, it looks like the low level activity doesn't get much below Lat 42.

Auroral glow can certainly effect the sky at your latitude, but it will be weak for the most part. I check http://www.spaceweather.com/ for auroral events because even though they rarely have a noticeable impact on sky conditions here at 45 degrees north, every so often...

Is it a big gray zone area? How deep into the blue zone do you observe? If you're going deep into a large area of the gray zone, and if you normally observe from the blue zone but close to the border with the green zone, then there is a chance that the difference will be noticeable. No guarantee, though.

On the other hand, I would say that you might as well try it at least once: sure, an extra two hours might be too much on a regular basis...but why wouldn't you try it once?

As others have posted above, the color zones are only a rough guide. Be aware that the Bortle Class of a certain location can change several times during the night. From the green zone locations I normally observe from, I have experienced Class 5, Class 4, and Class 3 conditions. One exceptionally clear night, I experienced Class 2 conditions from that green zone area for a few hours!

A single visit for a few hours to a location is NOT a true way to estimate how dark the location is on a regular basis. Only multiple visits to the same location under the same general conditions (no moon, clear skies, fully dark-adapted eyes) can give you a better idea of the "average" sky darkness of that location.

You could also just make a nice mini-vacation out of it and see other things in the area and that way if the sky conditions don't work out for you, you can still say you had a good time.

Starman - The statement made from the post you cite is, I'm afraid, decidedly off the mark with regard to accuracy when it is indicated that:

"Areas designated blue, grey or black are almost identical in appearance from 30° above the horizon to the zenith, in my experience. If fortunate enough to be observing from a clearing which is surrounded by tall, dense trees, which thoroughly block out any light domes that may be present on the horizon, one would be hard-pressed to discern much, if any, difference between these areas with one's eyesight alone."

The presence of light domes along the horizon impact the darkness of the entire sky to at least some degree, so simply blocking them out, together with hiding the lower 30* of sky, will not negate the differences between Bortle Classes 3, 2 and 1.

I would add that it is very unlikely that more than perhaps a handful of folks here have ever actually experience Bortle Class 1 skies. Believe me, if you ever have that opportunity, you'll have no doubt that decided differences exist between Bortle classes at the very lowest end of the scale.

Addressing the OP's question, the actual worth of driving considerably farther out from civilization to gain one or maybe two additional levels of darkness will depend critically on the nature of the observation you are attempting to make and perhaps the ultimate value that it may have to you and to others, at least in my book.

Mak17, that's dark than most place in Illinois! Enjoy as much as you can! Orlando, Disney, Miami, Tama Bay and many town growing to gain light pollution. I know Harmony (cant remember spell) is dark sky town near St Cloud but not that dark. Green zone is good in Illinois and hope someday to improve reduce light pollution everywhere!

MEE posts = "As others have posted above, the color zones are only a rough guide. Be aware that the Bortle Class of a certain location can change several times during the night. From the green zone locations I normally observe from, I have experienced Class 5, Class 4, and Class 3 conditions. One exceptionally clear night, I experienced Class 2 conditions from that green zone area for a few hours!

A single visit for a few hours to a location is NOT a true way to estimate how dark the location is on a regular basis. Only multiple visits to the same location under the same general conditions (no moon, clear skies, fully dark-adapted eyes) can give you a better idea of the "average" sky darkness of that location."

All of which is absolutely true. And it does point up that many here are addressing the Bortle scale in an incorrect manner. Its intent is more in the way of the evaluation and documenting prevailing sky conditions AT THE TIME the specific observation being reported was made, not necessarily to PREDICT what a given site experiences on a night-to-night basis. While such a use can indeed provide some indication of what can be anticipated it is no guarantee that a given night, even at an outstanding location, might not actually differ by one or even two classes depending on prevailing sky conditions.

I observe regularly in "blue" or "Bortle 3" sites in S. California, and occasionally in a grey or Bortle 2 site.I don't personally have an SQM meter, but from hearing others, a typical night at my blue sites are 21.3 and a 21.5 to 21.6 at the grey site.

To me there is a noticeable difference. For example, scanning the Milky Way, dark nebula jump out at you compared to just being able to see them. Dust lanes in M31 are more pronounced on average. The pesky very low contrast "bright" nebula like some of the Sharpless listings are a little easier. Faint globulars are easier. Everything really looks a little better at least.

I don't go to the grey zones often because I feel more tired at work on Monday morning with the longer drives. So it is a tough call for me personally on which is better overall.

From my experience, BrooksObs is correct about this. It is imperative to keep Bortle classes and color zones separate. I live in a blue zone and often observe in a gray zone. For decades I lived in a black zone. Is it worth driving two hours to go fron blue to gray. With a reasonable chance of equally good skies at both, I would say go...at least once, and see for your self. For me, it would be too far to go on a regular basis, although I will drive that for a black zone at high altitude---8000-10000 feet. All the caveats mentioned apply---skies can change in minutes. I have been in and observed under Bortle 1 skies, but I think they are pretty darn rare in North Americs. Exist but rare.

Thanks everyone for your feedback. It is a pretty big gray zone i guess. I normally observe with a green zone bordering the blue zone to the north but blue in every other direction. We normally can only see one lightdome to the north. Hoping to see no light domes as described on the cleardarksky website description of gray zone. I usually observe DSOs hoping to see more faint details. Just cleaned my mirror in anticipation. Only two more weeks....

Is your "gray zone" location still gray? Not that it really matters......

I would guess that several light pollution domes (likely small) would still be visible, unless you're going to one of those really big gray zones, such as in West Central Texas. Even then, I wonder if there's ANY place in the U.S. without a light dome of some sort.

... but it does make me wonder about the meaning of blue, gray and dark gray zones, or the reliability of Sky Quality Meters.

Simply: those colors DOES NOT represent the sky quality in the sense of Bortle. The Original Light pollution maps (by Cinzano and Falchi) were maps of the artificial light injected into the atmosphere. Other maps have been later produced, which counts for how much of this light becomes sky brightness. Moreover, the extinction plays an important role: a 21.5 sky with 0.2 mags extinction (such as on top of a mountain) is completely different than a 21.5 sky with 0.4 mags extinction (such as at the ground). In the former case the sky i brighter because of less attenuation and the amount of artificial light is much less (more signal to noise ratio).Lastly all maps, except the local ones which Cinzano mention, NELECT the screen effect of mountains. With no high mountains light may be shielded only by earth curvature. That is why on flat lands pollution domes are seen hundreds miles far. A 3000 meters mountain range may block lights like a 400 kms of flat lands.

One notable example is the Alps. These are "green" on the original maps. Are better on the maps that account for extinction but are even better in the reality.

Here is one example: http://oi42.tinypic.com/sc7upw.jpg Note that the sky background is greenish, which means that it nearly pristine because green is the color of the Airglow. At "only" 100 km there is a densely populated are with millions of people. That place would be Bortle 4 without the Alps. Instead it is Bortle ~2. BTW the week before Fabio Falchi at nearly the same location could spot M33 at ony 10° above the horizon. The SQM-L pointed to the big dipper read 21.89. The SQM pointed at the zenti (in Cygnus) read 21.64 (average of several readings).

40 km to the south this is the situation: http://oi40.tinypic.com/2ajvq05.jpg Here the "screen effect" is less and veils are lightened from lights 60 km to the south. This is light pollution (in fact it is red). The SQM pointed at the zenit read ~21.5.

Two other factors I feel impact the quality of a site. One is weather and number two related to that is season. Depending on a season a site can be brighter than it is during another season(s) depending on the glow of Milky Way itself. Also, weather is another factor and the conditions of prevailing winds. Here in northern Utah, fall and winter can have some of the best observing, if you catch new moon after a storm has vacated. Air is still at times, prevailing winds/jet stream will take a bump up to Montana and we just have some outstanding conditions for observing, cold, but still outstanding. However in Spring you can go to central or northern Utah, sometimes even into southern Utah and the jet stream often parks right above our state ushering in spring storms and making for nights of difficult observing conditions. So yes, Bortle and darkness are critical factors, but a slew of factors come into selecting an area to observe. I have a local one and a far one. I am very happy with both, even with both their weaknesses and strengths. I think many people here would love to observe three to four times a month where I do. Not all, but most.